The present invention relates to a method of detecting temperature using a semiconductor device and the circuit thereof which dispenses with a need for a reference voltage which needs to be adjusted. Particularly, the invention relates to a method of detecting temperature using a semiconductor device having a circuit comprising a semiconductor temperature sensor and an AD converter capable of being utilized in an integrated circuit such as a temperature compensated type crystal oscillator (TCXO) having a function of a temperature detector.
When temperature is detected in an integrated circuit to thereby provide a digital signal in correspondence with the temperature, there is generally constructed a constitution in which an output of a semiconductor temperature sensor 11 is converted by an AD converter 12 as shown by FIG. 10.
As the semiconductor temperature sensor 11, there is used a circuit in which a constant current source is connected to an emitter terminal of transistors connected in Darlington connection as shown by FIG. 11 (refer to Japanese Patent No. 2946306 or Japanese Patent Laid-Open No. 248962/1993).
In order to convert an output Vout of the sensor 11 into a digital signal by using the AD converter 12, as shown by FIG. 12, comparison voltage Vref produced from a certain reference voltage and the sensor output voltage Vout are compared by a comparator to thereby provide the digital value in correspondence with the sensor output voltage Vout (comparison system AD converter).
According to the conventional circuit shown by FIG. 12, the sensor output voltage is compared with the certain reference voltage. Although the reference voltage is generated by a reference voltage source such as bandgap reference, the voltage varies from reference to reference due to fabrication variations, and the output value of the AD converter varies accordingly. Therefore, in order to meet the need for a reference voltage source having high accuracy, adjustment by a trimming step after fabrication is required. That is, according to the conventional circuit, the absolute value of the reference voltage is utilized and therefore, there is needed an adjusting step for compensating for fabrication variances and there poses a problem that the circuit is complicated by that amount and the number of steps is increased.
It is an object of the invention to provide a semiconductor temperature detecting circuit with high accuracy and capable of ensuring absolute accuracy of detecting temperature with no need for a reference voltage to thereby dispense with steps of adjusting for fabrication variations, such as trimming.
In order to achieve the above-described object, according to the invention, there is constructed a constitution of comparing output voltages of two temperature sensors in place of a constitution of comparing output voltage of a temperature sensor and reference voltage.
That is, according to an aspect of the invention, there is provided a semiconductor temperature detecting circuit comprising a first and a second semiconductor temperature sensor, means for supplying different constant currents to the first and the second semiconductor temperature sensors, and means for detecting temperature based on a corresponding relationship between a ratio of output voltages of the first and the second semiconductor temperature sensors and the temperature.
In the circuit according to the aspect of the invention, there may be constructed a constitution in which the first and the second semiconductor temperature sensors are formed at portions of the same semiconductor substrate proximate to each other and there are respectively provided bipolar transistors connected in Darlington connection having differing numbers of stages for each sensor.
In the circuit according to the aspect of the invention, there may be constructed a constitution in which the means for detecting the temperature comprises a voltage dividing circuit for dividing the output voltage of the first semiconductor temperature sensor by a predetermined ratio, a comparator receiving an output voltage of the first semiconductor temperature sensor divided by the voltage dividing circuit as a first input and receiving an output voltage of the second semiconductor temperature sensor as a second input, and a processing circuit for detecting the temperature based on an output of the comparator and the corresponding relationship between the ratio of the output voltages of the first and the second semiconductor sensors and the temperature.
In the circuit according to the aspect of the invention, there may be constructed a constitution in which the voltage dividing circuit outputs divided voltages of the output voltage of the first semiconductor temperature sensor corresponding to a plurality of the predetermined ratios and the comparator successively receives the plurality of divided voltages at the first input and successively compares the first input with the second input.
Or, in the circuit according to the aspect of the invention, there may be constructed a constitution in which the dividing circuit outputs divided voltages of the output voltage of the first semiconductor temperature sensor according to a plurality of the predetermined ratios and there are present comparators corresponding in number to the plurality of output voltages of the dividing circuit which respective receive the plurality of divided voltages to the first inputs of the respective comparators and simultaneously compare the first inputs with the second input.
In the circuit according to the aspect of the invention, there may be constructed a constitution in which the processing circuit outputs the detected temperature as a digital signal.
In the circuit according to the aspect of the invention, there may be constructed a constitution in which the semiconductor substrate is of a conductive type of a P type or an N type.
Further, according to another aspect of the invention, there is provided a method of detecting temperature by a semiconductor device comprising the steps of providing a first and a second semiconductor sensor, supplying different constant currents to the first and the second semiconductor temperature sensors, calculating a corresponding relationship between a ratio of output voltages of the first and the second semiconductor temperature sensors and temperature, and detecting the temperature based on the corresponding relationship.
In the method according to the another aspect of the invention, there may be constructed a constitution in which the step of providing the first and the second semiconductor temperature sensors includes a step of providing bipolar transistors connected in Darlington configurations having differing numbers of stages of each sensor and which are provided on a same semiconductor substrate.
In the method according to the another aspect of the invention, there may be constructed a constitution in which the step of detecting the temperature further comprising the steps of dividing the output voltage of the first semiconductor temperature sensor by a predetermined ratio, comparing the divided output voltage of the first semiconductor temperature sensor as a first input and the output voltage of the second semiconductor temperature sensor as a second input, and detecting the temperature based on a result of the comparison and the corresponding relationship between a ratio of output voltages of the first and the second semiconductor temperature sensors and temperature.
In the method according to the another aspect of the invention, there may be constructed a constitution in which the step of dividing the output voltage further comprising the steps of dividing the output voltage by a plurality of the predetermined ratios, successively applying the plurality of divided voltages to the first input and successively comparing the first input with the second input.
Or, in the method according to the another aspect of the invention, there may be constructed a constitution in which the step of dividing the output voltage further comprising the steps of dividing the output voltage by a plurality of the predetermined ratios, and simultaneously applying the plurality of divided voltages to first inputs of comparators and simultaneously comparing the first input with the second input.
The two semiconductor temperature sensors according to the invention are respectively constituted by bipolar type transistors connected in Darlington connection. Current values of constant current sources of the two temperature sensors are set to values different from each other. Therefore, although output voltages V1 and V2 of the two temperature sensors change with regard to temperature T, temperature coefficients thereof differ as shown by a graph of FIG. 1. A change in a ratio V2/V1 of the two output voltages with regard to the temperature T is as shown by a graph of FIG. 2. As shown by the graph of FIG. 2, the voltage ratio V2/V1 is provided with temperature dependency.
Therefore, temperature can be detected by comparing and using the output voltages V1 and V2 of the respective temperature sensors. When the two sensors are constituted by elements having the same shape on the same chip and arranged at a short distance therebetween, there is achieved a tracking effect respectively indicating variations in characteristics in the same direction with regard to fabrication variations and when the output ratio V2/V1 of the two sensors is adopted, the variations in fabrication are canceled by each other and there is shown a temperature characteristic which is little affected by variations in fabrication. The output ratio V2/V1 is shown by the graph of FIG. 2.